A satellite broadcast tuner, a television receiver that has an internal satellite broadcast tuner, and so forth have an antenna level indication function that indicates the C/N (Carrier to Noise) ratio of a reception signal received from a satellite by an antenna. The antenna level indication function is used for example to adjust the direction of an antenna that is installed.
In the satellite broadcast, a parabola antenna that has a sharp directivity is used to receive a signal from the satellite. Thus, after the antenna has been installed, the direction of the antenna should be adjusted.
When the antenna is installed, the C/N ratio of the reception signal is displayed on the screen of for example a television receiver with the antenna level indication function. While the direction of the parabola antenna is being moved, the antenna level is checked so that the value of the antenna level becomes the maximum. After the direction of the antenna has been adjusted, when the value of the antenna level is equal to or larger than a predetermined C/N ratio, the satellite broadcast can be received.
The receiving apparatuses of the related art accomplishes the reception level indication function on the basis of the gain of an AGC (Automatic Gain Control) amplifier. In other words, an intermediate frequency amplifier stage of the satellite broadcast receiving apparatus has an AGC circuit that causes the level of the received signal to be constant. The AGC circuit detects the reception signal and the level thereof and sets the gain of the amplifier of the intermediate frequency amplifier stage corresponding to the level of the reception signal. Assuming that noise contained in the reception signal is white noise, since the signal level of the reception signal represents the C/N ratio, it can be detected from an output of the AGC circuit. The C/N ratio obtained from the output of the AGC circuit is indicated as an antenna level.
The receiving apparatus of the related art detects a C/N ratio with a detected output of the AGC circuit and indicates the C/N ratio as an antenna level. However, in particular, when a digital satellite broadcast receiving apparatus that uses a digital modulation system detects a C/N ratio with a detected output of an AGC circuit, even if an indicated antenna level is equal to or larger than a predetermined C/N ratio, the apparatus may not obtain a satisfying reception state.
In other words, an antenna that receives a digital satellite broadcast has an LNB (Low Noise Block Down Converter) that converts a reception signal of 12 GHz band into an intermediate frequency signal of 1 GHz band. The LNB often generates phase noise and parasitic oscillation. Since the digital satellite broadcast uses a phase modulation such as BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 8PSK (8-phase PSK), or the like, if the LNB generates phase noise and parasitic oscillation, the receiving apparatus does not obtain a satisfying reception state.
The receiving apparatus of the related art indicates an antenna level with a C/N ratio detected as an output of an AGC circuit. The apparatus detects the C/N ratio assuming that the output of the AGC circuit is white noise. Thus, the C/N ratio detected in the apparatus does not reflect phase noise. Thus, when the apparatus receives a digital satellite broadcast that has been phase modulated, even if the antenna level is equal to or larger than a predetermined C/N ratio, the apparatus may not be able to obtain a satisfying reception state.
If a satisfying reception state is not obtained due to phase noise and parasitic oscillation, when the phase of a demodulation circuit of the receiving apparatus is compensated (the bands of loop filters and a dumping factor are adjusted), the reception state can be improved.
However, the receiving apparatus of the related art does not indicate values that represents phase noise and parasitic oscillation. Thus, it is difficult to compensate the phase of the demodulation circuit. In other words, since the values of the phase noise and the parasitic oscillation are not indicated, while the picture quality of a reproduced picture is being evaluated, the phase of the demodulation circuit of the receiving apparatus should be compensated so that the picture quality becomes optimum. Adjusting the phase of the demodulation circuit while evaluating the picture quality needs skills.
A method that deals with phase noise is described in for example Japanese Patent Laid Open Publication No. 2000-13705.
The satellite broadcast receiving apparatus of the related art measures the C/N ratio of a reception signal with a detected output of an AGC circuit assuming that noise contained in the reception signal is white noise and indicates the C/N ratio as an antenna level. Thus, if phase noise and parasitic oscillation take place, even if the indicated antenna level is equal to or larger than the predetermined C/N ratio, the receiving apparatus may not obtain a satisfying reception state. In this case, it is difficult to identify which of the receiving apparatus side and the antenna side has a problem. Thus, the reception fault may not be correctly handled. As a result, the user may be confused with them.
When a satisfying reception state is not obtained due to phase noise and parasitic oscillation, although the phase of the demodulation circuit of the receiving apparatus may be compensated, since the receiving apparatus of the related art does not indicate the values of phase noise and parasitic oscillation, it is difficult to compensate the phase of the demodulation circuit. In addition, once the phase of the demodulation circuit has been compensated corresponding to the LNB that generates phase noise and parasitic oscillation, if the LNB is replaced with another one that has low phase noise and does not generate parasitic oscillation, the reception state may more deteriorate than with the former LNB.
Therefore, an object of the present invention is to provide a receiving apparatus and a C/N ratio indication method thereof that allow phase noise and parasitic oscillation to be detected, deterioration of the reception state due to phase noise and parasitic oscillation to be alerted, a reception defect to be properly handled, and the user to be prevented from becoming confused.